1. Field of the Invention
The present invention relates to an all optical gain-clamped amplifier, and more particularly, to an all optical gain-clamped amplifier which can keep the population inversion level of an amplifier which corresponds to variations in the number of input signal channels to a constant level by using stimulated Brillouin scattering (SBS) light which exhibits nonlinear characteristic to an input signal by employing a gain medium and a nonlinear mineral material of the third order nonlinearity, and therefore can automatically clamp a gain.
2. Description of the Related Art
In general, in an optical communications system, optical amplifiers are used to compensate for signal losses. In various all optical networks, signal losses vary according to variations in conditions due to circumstances such as faults, switching, adding and/or dropping.
FIG. 1 shows a schematic system block diagram of a conventional optical amplifier.
Referring to FIG. 1, a conventional optical amplifier comprises an optical wavelength selective coupler 11 for coupling an input optical signal by a wavelength selective coupling method, a gain medium 12 for amplifying a signal from the optical wavelength selective coupler 11 and outputting the amplified signal, and a pump 13 connected to the optical wavelength selective coupler 11 for causing a population inversion in the gain medium 12.
In the conventional optical amplifier having the above structure, when an input signal P.sup.in1.sub.sig /channel of n channels or an input signal P.sup.in1.sub.sig /channel of n-k channels is input into the gain medium 12 via the optical wavelength selective coupler 11, the gain medium 12 amplifies the input signal P.sup.in1.sub.sig /channel of n channels or the input signal P.sup.in1.sub.sig /channel of (n-k) channels and outputs P.sup.1.sub.out /channel or P.sup.2.sub.out /channel, respectively. At this time, the output signal P.sup.1.sub.out /channel of n channels can be expressed by GP.sup.in1.sub.sig /channel which is obtained by multiplying the input signal P.sup.in1.sub.sig /channel of n channels by a gain G, and the P.sup.2.sub.out /channel can be expressed by G.sup.1 P.sup.in1.sub.sig /channel which is obtained by multiplying the input signal P.sup.in1.sub.sig /channel of (n-k) channels by a gain G.sup.1. Thus, the gains G and G.sup.1 have different values. This means that the population inversion level of the gain medium 12 varies with variations in the level of a total input signal according to the variations in the number of channels of an input signal.
Thus, in the conventional optical amplifier, since the population inversion level varies with variations in the number of channels of an input signal, and therefore the signal output per each channel varies, it is difficult to design a system efficiently. As the complexity in all optical network, a major potential problem associated with the optical amplifier is a need for control of the gain due to circumstances such as faults, switching, adding and/or dropping of wavelengths. Transient effects as well as gain fluctuation result in degradation of signal quality in the surviving channel. So, they must be suppressed and a gain-clamped mechanism that can automatically clamp the gain per each channel of an amplifier according to the variations in the channel number of an input signal to a constant level is required. To date, the most promising candidate for the solution of these problems are based on the following two techniques. First, a lasing mechanism in the gain media either by using a couple of gratings at both ends or by forming a ring laser. Second, the feed back (forward) scheme where the optical power is measured and the pump power is adjusted to maintain appropriate population inversion level. The major drawback in the first technique is caused by the relaxation oscillation in the laser that prohibits switching near the relaxation oscillation frequency. In the second technique, optical signal power monitoring may be accompanied by information regarding existing number of signal channels in order to distinguish the change of input power levels and changes in number of signal channels or both thus making the system complicated. This may be an important issue in systems using pre-emphasis.